1. Field of the Invention
The present invention relates to a dielectric ceramic composition which is advantageously used in a laminated ceramic capacitor having an internal electrode formed of a base metal such as nickel or nickel alloy. The present invention also relates to a laminated ceramic capacitor which is formed from the dielectric ceramic composition and to a method for producing the capacitor.
2. Description of the Related Art
A laminated ceramic capacitor includes a laminate formed of a plurality of laminated dielectric ceramic layers and an internal electrode laminated therein. Recently, the internal electrode has been formed of an inexpensive base metal such as Ni rather than an expensive noble metal such as Ag or Pd in order to reduce cost.
When the internal electrode is formed of a base metal such as Ni, the electrode must be fired in a reducing atmosphere so as to avoid oxidizing the base metal. However, when fired in a reducing atmosphere, a ceramic formed of barium titanate is disadvantageously reduced to become semiconductive.
In order to solve this problem, there has been developed a technique for preventing reduction of dielectric materials by modifying the ratio of the barium sites/titanium sites in the barium titanate solid solution such that it exceeds the stoichiometric ratio (Japanese Patent Publication (kokoku) No. 57-42588). Through this technique, a laminated ceramic capacitor having an internal electrode formed of a base metal such as Ni can be put into practical use, and production of such capacitors has increased.
With recent advances in development of electronics, miniaturization of laminated ceramic electronic elements has progressed rapidly. In the field of laminated ceramic capacitors, trends towards miniaturization and increased capacitance are also noticeable. In addition, laminated capacitors must have an electrostatic capacity that is higher and have a lower dependence on temperature. Thus, a variety of materials having high dielectric constant and excellent temperature-related characteristics have been proposed and put into practical use.
Thus far, all the proposed materials comprise BaTiO3 as a primary component and a rare earth element, which is diffused into BaTiO3 grains during sintering, as an additive. Grains that constitute the obtained sintered compacts are known to have a core-shell structure comprising a core portion containing no diffused additive component and a shell portion containing the diffused additive component. Therefore, the combination of the core portion and the shell portionxe2x80x94which differ according to the temperature dependence of the dielectric constantxe2x80x94provides a composition whose dielectric constant has a low dependence on temperature.
These materials realize laminated ceramic capacitors having high electrostatic capacity and low dependence on temperature, and thus have greatly contributed toward broadening of the market.
However, the core-shell structure, which is attained through sintering of ceramics and control of diffusion of the additive component, also involves a disadvantage. Specifically, as sintering progresses the additive component diffuses excessively to fail to provide low dependence on temperature, whereas insufficient sintering results in poor reliability. Achieving control of sintering and diffusion is relatively difficult with the above-described materials, causing undesirable variation in the temperature dependence of dielectric constant.
Furthermore, in order to satisfy demand for miniaturization and high electrostatic capacity, dielectric ceramic layers formed in a laminated compact must be made thinner and the laminates must comprise a greater number of layers. However, when the ceramic layers become thin, a smaller number of ceramic grains are included between internal electrodes and this remarkably deteriorates the reliability of the capacitor. Thus, the decrease in the thickness must be limited. Therefore, development of materials having high reliability and exhibiting low variation in dielectric constant with temperature and electric field must be achieved through a decrease in the size of ceramic grains.
Meanwhile, many electronic elements such as those used in automobiles are used in a high-temperature environment, and therefore those whose characteristics remain stable at high temperature are desired. Specifically, there is desired a laminated ceramic capacitor of high reliability and having a dielectric constant having a low temperature dependence at higher temperature (e.g., 150xc2x0 C.).
However, the sinterability of conventional materials having a core-shell structure and diffusion of an additive component increases as the BaTiO3 grains become smaller, which causes difficulty in maintaining low temperature dependence characteristics. Since BaTiO3 exhibits a large variation in dielectric constant at high temperature (e.g., 150xc2x0 C.), maintaining a dielectric constant having low temperature dependence up to high temperature is relatively difficult.
As described hereinabove, according to the state of the art, realization of a sufficiently thin laminated ceramic capacitor and a dielectric constant of sufficiently low temperature dependence by use of a material having a core-shell structure is difficult.
In view of the foregoing, an object of the present invention is to provide a dielectric ceramic composition to solve the above-described problems. Another object of the invention is to provide a laminated ceramic capacitor produced from the composition. Still another object of the invention is to provide a method for producing the ceramic laminated capacitor.
In short, the dielectric ceramic composition according to the present invention is a material that does not have a core-shell structure formed through diffusion of an additive component, i.e., a material whose temperature-dependent characteristics and reliability do not depend on diffusion of an additive component. A laminated ceramic capacitor produced from the dielectric ceramic according to the present invention satisfies the B characteristics specified by JIS specifications and satisfies the X7R and X8R characteristics specified by EIA specifications.
In one aspect of the present invention, there is provided a dielectric ceramic composition comprising a complex oxide containing Ba, Ca, Ti, Mg and Mn as metal elements.
In another aspect of the present invention, there is provided a dielectric ceramic composition represented by the following formula: {Ba1-xCaxO}mTiO2+xcex1MgO+xcex2MnO wherein 0.001xe2x89xa6xcex1xe2x89xa60.05; 0.001xe2x89xa6xcex2xe2x89xa60.025; 1.000 less than mxe2x89xa61.035; and 0.02xe2x89xa6xxe2x89xa60.15.
Preferably, the dielectric ceramic composition according to the present invention further contains a sintering aid in an amount of about 0.2-5.0 parts by weight based on 100 parts by weight of the remaining components of the dielectric ceramic composition. Preferably, the sintering aid comprises SiO2 as its primary component.
In another aspect of the present invention, there is provided a laminated ceramic capacitor which is formed of a dielectric ceramic composition comprising a complex oxide containing Ba, Ca, Ti, Mg and Mn as metal elements.
More specifically, the laminated ceramic capacitor includes a laminate formed of a plurality of dielectric ceramic layers and further includes a plurality of external electrodes provided at different positions on side faces of the laminate, wherein each of a plurality of internal electrodes are formed along an interface between two adjacent dielectric ceramic layers such that each of the internal electrodes has one end exposed to one of the side faces so as to establish electric contact with one of the external electrodes. The dielectric ceramic layers are formed of the above-described dielectric ceramic composition. The internal electrodes of the laminate ceramic capacitor preferably contain Ni or an Ni alloy.
In yet another aspect of the present invention, there is provided a method for producing a laminated ceramic capacitor comprising the following steps:
a step for preparing a mixture comprising a compound represented by {Ba1-xCaxO}TiO2, an Mg compound, and an Mn compound;
a step for fabricating a laminate by laminating a plurality of ceramic green sheets containing the mixture and a plurality of internal electrodes each formed along an interface between two adjacent ceramic green sheets such that each of the internal electrodes has one end exposed to one of the side faces;
a step for firing the laminate; and
a step for forming a plurality of external electrodes on each side face of the laminate such that the one end of each of the internal electrodes exposed to the side face is electrically contacted with one of the external electrodes.
Preferably, in the method for producing a laminated ceramic capacitor, the content of an alkali metal oxide present as an impurity in the compound represented by {Ba1-xCaxO}TiO2 is about 0.03 wt. % or less.
Preferably, the compound represented by {Ba1-xCaxO}TiO2 has an average particle size of about 0.1-0.8 xcexcm.
The average particle size of the compound represented by {Ba1-xCaxO}TiO2 may be about 0.1 xcexcm -0.3 xcexcm, or may be more than 0.3 xcexcm but not more than about 0.8 xcexcm. More preferably, the maximum particle size of the compound is about 0.5 xcexcm or less for the former case and is about 1.0 xcexcm or less for the latter case.
In the method for producing a ceramic capacitor according to the present invention, the ratio of (average grain size of dielectric ceramic product)/(average particle size of provided starting material powder), which is represented by R, is preferably about 0.90-1.2.
In the above-described aspects of the present invention which relate to the compositions and method for producing a laminated ceramic capacitor, the dielectric ceramic compositions may further contain a rare earth element, which is represented by RE. RE is preferably selected from the group consisting of Y, Gd, Tb, Dy, Ho, Er and Yb.